1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, Advanced Mobile Phone Services (AMPS), digital AMPS, Global System for Mobile Communications (GSM), Code Division Multiple Access (CDMA), Local Multipoint Distribution Systems (LMDS), Multipoint Multichannel Distribution Services (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a Public Switched Telephone Network (PSTN), via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage (demodulator). The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. As used herein, the term “low IF” refers to both baseband and intermediate frequency signals. A filtering stage filters the low IF signals to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard. Alternate designs being pursued at this time further include direct conversion radios that produce a direct frequency conversion often in a plurality of mixing steps or stages.
Phase locked loops (PLLs) are becoming increasingly popular in integrated wireless transceivers as components for frequency generation and modulation. PLLs are typically used for one of a variety of functions, including frequency translation to up-convert a baseband (BB) signal to an intermediate frequency (IF) or to up-convert a baseband or IF signal to RF prior to amplification by a power amplifier and transmission (propagation). PLLs allow for a high degree of integration and, when implemented with the appropriate amount of programmability, can form a main building block for modulators that operate over a wide range of frequencies. Typically, a baseband processor produces a baseband digital signal that is converted to a continuous waveform signal by a digital-to-analog converter (DAC). The continuous waveform signal constitutes the analog baseband signal that requires up-converting to IF and then RF.
A class of PLL-based transmitters, known as translational loops, has become particularly popular. Briefly, in a translational loop, the desired modulated spectrum is generated as some low IF or at DC and then is translated to the desired RF using a PLL. In applications with non-constant envelope modulation, a parallel path for amplitude variation modulates the output power amplifier to generate the desired amplitude variation. One problem with current translational loops, however, is that reference signals, and especially IF reference signals, couple to an output VCO of the translational loops through undesired circuit paths. This phenomenon is referred to as “reference feedthrough” or “IF feedthrough” and is particularly prevalent in low voltage supply CMOS technologies optimized for digital processing. Many wireless communication standards, for example the GSM standard for cellular communications, impose strict limits on the spurious emissions of a given transmitter. Since reference feedthrough manifests itself as spurious emission in the RF output, many design efforts go into ensuring adequate attenuation of the reference feedthrough when designing a translational loop type transmitter for GSM. For example, when employing a 26 MHz reference signal, the GSM standard limits the reference feedthrough to −79 dBm (measured over a 100 kHz per Newton bandwidth). Normalized to a transmitter with an output power of +33 dBm (a typical GSM specification), the limitation on the reference feedthrough is −112 decibels relative to the carrier (dBc).
The closed loop PLL signal filter of the translational loop can be used to attenuate the reference feedthrough since this is an input referred to as? noise source. However, as it turns out, in CMOS technology the level of reference feedthrough is typically so significant that the closed loop PLL signal filter must be made very narrow, e.g., a few hundred kilo-hertz (kHz), (KHz per Newton) in order to attenuate the reference feedthrough to an acceptable level. This, however, in turn, imposes a large distortion on the transmitted signal and causes the transmitter to fail the modulation accuracy requirements of GSM.
For these and other reasons, radio circuits are being developed with many tradeoffs. On one hand, a need exists for transceivers that provide fast response to incoming signals. A need also exists, however, for transceivers that adequately filter noise, feedthrough, harmonics and other undesirable signals in the signal path.